Detection and Isolation of Differentially Expressed Genes by PCR

The approx 3�10⁹ basepairs that comprise a human genome are believed to contain at least 100,000 different genes (1 ). Many of these genes and the proteins that they encode have been isolated and identified, but the vast majority of these genes have yet to be characterized. A typical mammalian cell is believed to express approx 15,000-25,000 different proteins. A significant percentage of these proteins are present in virtually all cell types and they are generally designated as “housekeeping genes.” These include many of the various proteins involved in energy production, cellular biosynthesis, or the regulation of cell growth and division. The remainder of the proteins that the genome is capable of encoding are those that are differentially expressed. The expression of these proteins is often restricted to a few cell types or even just one particular cell type. For example, genes that are expressed only in a muscle or a kidney cell, or those that are found in a T or B lymphocyte, but not in both types of cells are differentially expressed.